1. Field of the Invention
The invention concerns a pneumatic suspension leveling system for vehicles, in particular for motor vehicles, with the characteristics of the introductory clause of claim 1. Such a pneumatic suspension leveling system is known to the art, for example, from DE 38 08 395 A1.
2. Description of Related Art
Pneumatic suspension leveling systems of the kind designated above are advantageously equipped with a central pressure accumulator in order, for example, to make short-term, quick level lifts possible for the vehicle for which the maximum lifting flow of the compressor used does not, as a rule, suffice. Moreover, with the aid of such a central pressure accumulator lifting actions can be carried out without the use of a compressor, whereby, on the one hand, the compressor can remain turned off and is consequently spared and, on the other, level adjustment can respond considerably faster because a compressor warmup phase is eliminated.
The compressed air stored in advance in such a central pressure accumulator is at a pressure level which, in any case, must be higher than the air pressure in the pneumatic suspension leveling elements assigned to the vehicle's wheels when the vehicle is fully loaded. In order to be able to constantly guarantee this pressure level, the pressure in the central pressure accumulator must be monitored, for which purpose an accumulator pressure sensor is provided.
An electronic control device of the pneumatic suspension leveling system implements in this connection, among other things, pressure regulation of the prevailing central pressure accumulator air pressure. At the same time air pressure in the central pressure accumulator is expediently maintained between an upper cutoff pressure and a lower cutoff pressure. In this connection, the lower cutoff pressure corresponds to the minimum pressure with which a lifting action can still be performed for the fully loaded vehicle. The upper cutoff pressure is the maximum pressure up to which the central pressure accumulator can be charged by the compressor. The upper cutoff pressure results, for example, from the functional capacity of the compressor used.
If, for example in the course of a vehicle leveling lift, air pressure in the central pressure accumulator falls below the lower cutoff pressure, the control device induces a recharging or refilling of the central pressure accumulator by the compressor. Such filling or charging action is ended as soon as the air pressure in the central pressure accumulator reaches its upper cutoff pressure.
Usually, with pneumatic suspension leveling systems of the kind designated at the outset, electrically driven, single-stage piston compressors or piston pumps are used. The volume flow delivered by such an electric compressor to a considerable extent is dependent on the respective ambient pressure and back pressure in the central pressure accumulator. Vehicles equipped with a pneumatic suspension leveling system of the kind designated at the outset (e.g., trucks and their trailers or busses) should ordinarily operate to full functional capacity in a range of elevation between 0 meters and 4,000 meters above sea level, which corresponds to an ambient pressure range of approximately 0.625 bar (4,000 meters above zero sea level) to approximately 1 bar (0 meters above zero sea level).
As a consequence of this relatively great pressure range for ambient pressure, the maximum air pressure attainable by the compressor used to charge the central pressure accumulator at low ambient pressure, i.e., at great elevations, is available to a considerably lesser extent at lower elevations than at great ambient pressure, i.e., at sea level. Trials have shown that with a specific compressor type at an ambient pressure of 1 bar a maximum charging pressure for the central pressure accumulator of 18 bar is attainable, while the maximum pressure attainable at an ambient pressure of 0.625 bar with the same compressor type is just 13 bar. In order that reliable charging of the central pressure accumulator can be guaranteed for this compressor type also at an ambient pressure of 0.625 bar, the upper cutoff value for the air pressure in the central pressure accumulator must be set to a value smaller than 13 bar. This means, however, that for the overwhelming majority of all operating conditions in which the vehicle equipped with a pneumatic suspension leveling system of the kind designated at the outset finds itself at an elevation lower than 4,000 m above sea level, the functional capacity of the compressor and, consequently, the advantages resulting from the use of a central pressure accumulator can be taken advantage of only to a reduced extent.